Bradford Dillman was originally unhappy with his character's 2D nature, and asked writer John Sayles why his character was so thin. Sayles responded that Roger Corman regularly did not use good actors in his film, so he deliberately didn't elaborate on characters. But since Dillman was a "real" actor, he was more than happy to enhance his character's depth.
A waitress from the Holiday Inn where the director and crew were staying stood in for Heather Menzies during the topless shots because Menzies was concerned that her husband might not approve of the nude scene.
The novelization fills in some of the background about Paul's character. His wife left him which is why he's such a hard drinker. The only explanation given is that she couldn't stand the sight of him. He doesn't write or try to contact his daughter, something Laura the camp councilor resented him for. She later changed her mind about Paul after saving (some of) the kids from piranha. Including his daughter. And the main reason Dumont thinks he's drunk over the phone is because during one visit to the camp, Paul drunkenly threw Dumont in the river when he told him to leave.
The novelization also fills in some trivia about Maggie. When her boyfriend just walked out on her, she hired a skip-tracer to find him. She got so hooked on the process she decided to become one herself. By the time she caught up with her ex, she wasn't interested anymore in why he left.
Piranha (1978) had some documented production problems. There were last-minute cast changes, underwater cameras that kept breaking down, union woes, and unusable second unit footage. It still went on to become one of New World Pictures' biggest hits.
The Aquarena Springs and Resort theme park (located in San Marcos Springs in San Marcos, TX), which opened in 1951, was purchased and became the property of Texas State University - San Marcos (formerly Southwest Texas State University). The amusement park ceased operations in 1996 and the facility is known as Aquarena Center (an environmental learning center).
When the guard at the dam is watching a commercial promoting the resort at Lost River Lake, originally some of Dumont's campers were part of the promotion, but this was cut from the movie. Another scene cut was Dumont accidentally appeared in the background of the commercial, ruining the shot. He was ordered off the premises by Buck Gardener.
According to Joe Dante in the DVD commentary originally released in 2004, the noise that the piranhas make underwater are the sounds of dental drills. While he didn't go into anymore detail than that, they could the put the edge of the drills into water and an underwater mic captured the sounds when they were turned on. Another possibility is the drills were turned on above water and recorded from underwater.
The film was designed to jump on the huge success of Jaws (1975). Roger Corman was known for producing films that were veiled imitations of more successful movies. But it still took a further three years to raise enough money to make the film. By which time Jaws 2 (1978) had already been made.
The trivia items below may give away important plot points.
Belinda Balaski's death scene was shot in a pool. Thirty fake piranha were attached to Balaski's body with gaffer's tape and a bunch of crew members pulled her into the deep end of the pool with ropes to make it look like her character was sinking into the water. Moreover, Roger Corman demanded a reshoot of Balaski's death scene because he thought the first version didn't have enough blood in it.
The boy on the canoe originally didn't emerge unscathed like he does in the film. He suffered severe lacerations on his arms but this doesn't appear in the film. He is however in a state of shock because of his father's death. He even thinks Hoak is his father, which is why he keeps calling him Daddy when Paul and Maggie dump his body overboard.